1. Field of the Invention
The present invention relates to a substrate processing apparatus and a substrate processing method for performing an etching process, specifically by immersing a substrate, such as a semiconductor wafer, a glass substrate for liquid crystal display, a glass substrate for photomask or a substrate for optical disk, on which a silicon oxide film and a silicon nitride film are formed, in a phosphoric acid solution to selectively etch the silicon nitride film.
2. Description of the Background Art
An etching process is a very important process step for pattern formation in manufacturing semiconductor devices. In recent years, especially, with high performance and high integration of semiconductor devices, required is an etching process on a substrate on which a silicon nitride film (Si3N4 film) and a silicon oxide film (SiO2 film) are formed, to selectively etch the silicon nitride film so that the silicon oxide film should be left on the substrate. As a method of selectively etching the silicon nitride film, an etching process using a phosphoric acid solution (H3PO4+H2O) of high temperature (150° C. to 160° C.) as an etching solution is disclosed in Japanese Patent Application Laid Open Gazette No. 2003-224106. Specifically, a plurality of substrates on each of which a silicon nitride film and a silicon oxide film are formed are immersed in a processing bath in which a phosphoric acid solution of high temperature is pooled, to selectively etch the silicon nitride film. From a viewpoint of the characteristics of the phosphoric acid solution, however, a very small amount of silicon oxide film is etched.
When a process of etching a silicon nitride film is performed by using a phosphoric acid solution, usually, siloxane is generated. “Siloxane” is a generic name for a family of organic or inorganic compounds whose main ingredients are silicon (Si) and oxygen (O). Siloxane generated in an etching process accumulates in an etching solution as an extraneous matter. When the concentration of siloxane in an etching solution is too low, the etching speed of silicon oxide film becomes higher and the etching selection ratio of silicon nitride film becomes lower. On the other hand, when the concentration of siloxane in the etching solution is too high, the siloxane adheres to the processing bath or clogs a filter. Therefore, when the etching process is performed by using the phosphoric acid solution, it is important to adjust the concentration of siloxane in the etching solution in an appropriate range in accordance with the purpose of processing.
Since the concentration of siloxane in the etching solution is naturally low immediately after inputting a new phosphoric acid solution into a processing bath for the change of solution or the like, a method (seasoning) for increasing the concentration of siloxane, in which an etching process is performed on substrates not to be processed, on each of which the silicon nitride film is formed, has been conventionally performed in order to adjust the concentration into the appropriate range.
This method, however, has a problem that the down time of the apparatus is too long. Therefore, there is another possible method in which an additive containing siloxane is inputted into the phosphoric acid solution in advance, but this method arises a problem that it is impossible to keep the etching characteristics of the phosphoric acid solution constant because of decomposition or deterioration of the additive.